Associative memories including mos transistors

ABSTRACT

An associative memory comprises at least one memory cell constituted by two MIOS transistors having hysteresis gate threshold voltage characteristics. The MIOS transistors are combined such that for a gate voltage of a selected value, one MIOS transistor is in ON state whereas the other OFF state. These states correspond to a binary &#39;&#39;&#39;&#39;1.&#39;&#39;&#39;&#39; On the other hand, the states where said one MIOS transistor is in OFF state and the other ON state correspond to binary &#39;&#39;&#39;&#39;0.

United States Patent Nojima et al. I

[ Nov. 5, I974 ASSOCIATIVE MEMORIES INCLUDING MOS TRANSISTORS [56] References (Jited [75] Inventors: Isao NojimatKeikichi Tamaru; Tai UNlTED STATES PATENTS Sate, all of Yokohama. Japan 3,750,115 7/1973 Mundy 340/173 AM 73 Assi nee: T k 0 Sh'b Ele t C Ltd., 1 g g i i g j 1 "c o Primary Examiner-Terrell W. Fears Attorney, Agent, or Firm-Kemon, Palmer & [22] Filed: Aug. 30, 1973 Estabrook [2!] Appl. No.: 393,037

Related US. Application Data L ABSTRAQT t l t n associa we memory compr1ses a eas one mem- [63] Commuauon of March 1972' ory cell constituted by two MlOS transistors having hysteresis gate threshold voltage characteristics. The [30] Forelgn Apphcamn Pnomy Data MIOS transistors are combined such that for a gate Mar. 25, 1971 Japan 46-16834 voltage f a selected value one 10 transistor is i ON state whereas the other OFF state. These states [52] US. Cl. 340/173 AM, 307/238, 307/279, correspond to a binary o the other hand, the 340/173 R states where said one MlOS transistor is in OFF state i 'gz' z' 3 5 and the other ON state correspond to binary 0. 307/238 279 5 Claims, 10 Drawing Figures 2A 2B R v ,4 1 d L J l-. 1

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WRITING INTERROGATION 1 o o 1|NTERROGATION INFORMATION -4V 0 0 4V 0 "4V -4V 0 ASSOCIATIVE MEMORIES INCLUDING MOS TRANSISTORS This is a continuation of application Ser. No. 237,455, filed Mar. 23, 1972.

BACKGROUND OF THE INVENTION This invention relates to an associative memory including MOS (metal oxide semiconductor) transistors, and more particularly to a memory device including MIOS transistors whose gate threshold voltages have hysteresis characteristics.

In an electronic computor, for example, it is necessary to determine whether a desired information is contained or not in a plurality of informations stored in a memory section of the computor by comparing the desired information with an interrogation information for reading out the information corresponding to the interrogation information thereby processing the read out information. An associative memory is important for attaining this object. An associative memory including flip-flop circuits containing MOS transistors has been proposed. However, this known type of the associative memory has the following defects.

First, since it is necessary to use from six to eight MOS transistors to constitute each one bit associative memory cell it is necessary to use a large number of MOS transistors in order to fabricate a large capacity associative memory.

for applying a control voltage upon respective gate electrodes of the pair of MIOS transistors, interrogation means including an interrogation line adapted to apply an interrogation voltage upon the respective first electrodes of the pair of MIOS transistors, and associative read out means including output lines commonly connected to the respective second electrodes of the pair of MIOS transistors for producing outputs corresponding to the interrogation voltage.

The gate electrodes of two MIOS transistors constituting the one bit memory cell may be commonly connected to a single word line. Alternatively, different work lines may be connected to different gate electrodes.

The first electrode of each MIOS transistor is connected to an interrogation line which may be used as a sense line or a digit line. The second electrodes of the pair of MIOS transistors are commonly connected to a coincidence or read out line (or matching line) on which appears a signal indicating whether an interrogation information supplied from the interrogation line coincides or not with the information stored in the one I bit memory cell.

Second, since flip-flop circuits are used, the contents of the memory are volatile or destroyed whenever the operating voltage is removed.

Third, since the digit lines and the interrogation lines of the associative memory are provided independently it is necessary to use a plurality of jumper lines for connecting them to other circuits or conductors whereby the memory device as a whole becomes greatly complicated.

Fourth, it is impossible to simultaneously read out a plurality of words because the memory contents of other words connected to the read out bit lines are destroyed.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved associative memory capable of eliminating any one and all of said defects.

Another object of this invention is to provide an improved associative memory characterized in that each one bit memory cell is constituted by two MIOS transistors, that the memory is non-destructive, that the same line can be used as a digit line as well as an interrogation line, and that the contents of the memory are not destroyed even when a plurality of words are read out simultaneously.

The associative memory of the invention comprises at least one bit memory cell constituted by a pair of MIOS transistors each having first and second electrodes and a gate electrode, the threshold value of the gate voltage having a hysteresis characteristic corresponding to the voltage impressed upon the gate electrode. Said each MIOS transistor takes an upper or lower value of threshold voltage according to said impressed gate voltage. The pair of MIOS transistors are combined such that for a selected gate voltage of a value intermediate the upper and lower values, one MIOS transistor is in ON state, whereas the other OFF state. The associative memory further comprises means As is well known-in the art, a MOS transistor having a threshold gate voltage hysteresis characteristic can be fabricated by constructing the gate insulator layer as multilayer films. The MOS transistor having such a construction is generally termed a MIOS transistor (metal insulated oxide silicon transistor) and the MIOS transistor whose insulator layer is constructed as two layers consisting of a silicon nitride (Si N,) layer and a silicon oxide (SiO layer is termed a MNOS (metal nitride oxide silicon) transistor. When a gate voltage of a relatively large absolute value is applied to the MIOS transistor of these types and then the gate voltage is reduced to zero these MIOS transistors will have a threshold voltageof upper or lower value dependent upon the polarity of the impressed voltage. A pair of MIOS transistors of such characteristics are combined into a one bit associative memory cell (there are three combinations, one including two MIOS transistors of the same conductivity type and having different type hysteresis characteristics, the second including two MIOS transistors of the different conductivity type) in such a manner that for a gate voltage of a value intermediate said upper and lower values of threshold voltage one of the MIOS transistors is in ON state and the other OFF state. The informations can be stored in the memory by making the states wherein one MIOStransistor is ON and the other is OFF to correspond to a binary l and the states wherein said one transistor is OFF and the other is ON to a binary 0.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a one bit memory cell comprising a combination of p-channel MIOS transistors and nchannel MOS transistors;

FIG. 2A shows a sectional view of the-one bit memory cell shown in FIG. 1 taken along a line 2A-2A;

FIG. 2B shows a sectional view of the one bit memory cell shown in FIG. 1 taken along a line 2B-2B;

FIG. 3 is a plot showing the gate voltage hysteresis characteristics of the MIOS transistors shown in FIGS. 2A and 2B;

FIG. 4 is a circuit diagram of an associative memory comprising four one bit memory cells shown in FIG. I;

3 4 FIG. 5 is a sectional view showing a modified emcome ON at a gate voltage of V -2V. On the other bodiment comprising a combination of a p-channel hand, once a voltage of 30V is applied to the gate MIOS transistor and an n-channel MOS transistor; electrode of transistor Rb, this transistor will become FIG. 6 is a plot showing the gate voltage hysteresis OFF at a voltage of V V and become ON at a characteristics of two MIOS transistors of the same 5 gate voltage of V 10V.

conductivity type but having different type hysteresis In the case of the n channeI MIOS transistor Ra charactenstlcs; when the gate voltage V is varied in the order of FIG. 7 shows a circuit diagram of an associative 0v 30 as shown by a dotted Iine curve, I fyt P hy the wrltihg meehs thereof threshold voltage Vth will vary along a curve a-b'-c-d wherein each one bit memory cell is constituted by two 10 whereby the threshuld voltage varies f 3 to Q ttehslsters of the e conductlvlty p but -1 1V. On the other hand, when the gate voltage is varhavmg dlfferent yp y t s characterlstlqs; ied in the order of 0v +30v 0v, the threshold volt- FIG. 8 shows a connection diagram of the interrogaage V h ill vary along a curve d-e-j-a whereby the tion means utilized in the associative memory shown in threShOId voItag varies from IV to Accord 7; l5 ingly, once a gate voltage of +30 V is impressed upon FIG. 9 s hows a connection diagram of the re transistor Ra this transistor Ra will become ON at a means utilized in the associative memory shown in FIG.

gate voltage of V 2 3V and become OFF at a gate and voltage of V 3V. Similarly, once a gate voltage of DESCRIPTION TIIE PREFERRED -V is impressed upon the gate electrode of transistor EMBODIMENTS Ra, this transistor Ra will become ON at a gate voltage o f V 1 1V and become OFF at a gate voltage of Referring now to the accompanying drawings, the

one bit associative memory cell R comprises an n- I channel type first MIOS transistor Ra and a p-channel Fehowlhg table shows the ON'OFF conditions of type econd transistor which are paced apart 25 tI'aIlSIStOX'S Ra and when gate voltages Of are and are secured to an insulator 3. Respective n regions fltstly impressed and then gate Voltages of 0V and of the n-channel MIOS transistor Ra are provided with are pp The Voltage y be y value a first electrode 4 and a second electrode 5, respectween the p e and lower hmlts of the threshold Volt tively, and a SiO layer 7 having a thickness ofabout 20 age that this Voltage Is not hmited to 5V ON OFF OFF ON angstroms overli'es respective n regions and a p region 40 As can be noted from this table, transistors Ra and 6. A Si N layer 8 having a thickness of about 500 ang- Rb assume opposite ON OFF states for the same gate stroms is applied on the layer 7, as shown in FIG. 2A. voltage V 5V. For this reason, it is possible to make In the same manner, respective p regions of the pthe ON state of the p-channel transistor and the OFF channel MIOS transistor Rb are provided with a first state of the n-channel transistor to correspond to the electrode 4 and a second electrode 5, respectively, and writing of a binary l and to make the OFF state of the a SiO layer 7 having a thickness of about 20 angstroms p-channel transistor and the ON state of the n-channel overlies respective p regions and n region 6'. A Si N transistor to correspond to the writing of a binary 0.

layer 8 is applied on the layer 7 as shown in FIG. 2B. This means that it is necessary to apply a voltage of As shown in FIG. 1, a common gate electrode 9 is apl:3 I or E OV upon thggateeleetrode for w riting.

P 0h layets 8 of both transistors Re and Layets FIG. 4 shows the connection diagram of one example 7 and 8 y he made of 2 3 0r at for p of the associative memory comprising a plurality of one ther one of the first and Second ele 4 and 5 is bit associative memory cells described above. In this used as the source electrode whereas the other as the fi ll R a d R rat t tit t one d drain electrode. It IS well kIlOWll in the art that the of two bits whereas ells R3 and R4 cooperate to constithreshold voltage Vth of these MIOS transistors takes t t th t bit d Th t l t d 9 f th the form of a hysteresis curve as shown in 3 in transistors of respective cells R, and R are commonly which the abscissa represents the gate voltage V and connected t a d h W whereas h gate l the ordinate the threshold voltage Vth. In the case of tr des of the transistors of cells R and R are comthe p-channel OS transistor When the g Voltmonly connected to the other word line W A digit line g a is Varied in the Order Of the D is connected to the first electrodes 4 of the nthreshold voltage Vth varies as ShO y a Curve channel transistors of cells R and R A digit line 5 is d. In Other O S, the threshold voltage varies fro connected to the first electrodes 4 of the -channel -2V to lOV. On the other hand, when the gate volttransistors of cells R and R A digit line 2 is conage V is varied in the order of 0V(+30V)0V, then nected to the first electrodes 4 of the n-channel transisthe threshold voltage Vth varies along a curve d-e-f-a tors of cells R and R whereas a digit line D is conwhereby the threshold voltage varies from l0V to nected to the first electrodes 4 of the p-channel transis- -2V. Accordingly, once a voltage of +30V is applied he tors of cells R and R The second electrodes 5 of the to the gate electrode of transistor Rb, this transistor will transistors of cells R, and R are commonly connected become OFF at agate voltage of VG 2V and beto one associative read out line (matching line) 8,,

whereas the second electrodes 5 of respective transistors of cells R and R, are commonly connected to the other associative read out line 8,. Read out lines S, and S, are grounded through load r e sistors L, respectively. Each one of the digit lines D,, D,, D, and D also acts as an interrogation line.

Writing of an information is performed in the following manner. First a voltage of 30V is applied to each of the word lines W, and W, for the purpose of clearing the memories of respective cells. At this time, a O is stored in each cell as shown in the table described above (that is the state wherein the p-channel transistor is OFF and the n-channel transistor is ON at V,,=5V). To write a l in cells R, and R,, a voltage of +30V is impressed upon the word line W,. Then, as shown in the table, the n-channel transistor will become OFF and the p-channel transistor at V SV, thus assuming a state of 1.

To interrogate whether a l is stored or not in the cell R,, 4V is applied to interrogation line D,, V to interrogation line D,, V to word line W, and 0V to word line W If the cell R, has been storing a 1, no output will appear on read out line S, notwithstanding the fact that the p-channel transistor is now being corgluctive because 0V is applied to the interrogation line D,. Furthermore, since the n-channel transistor is OFF, no output will appear on read out line S, notwithstanding the fact that 4V is applied to interrogation line D,. In other words, no output appears on the read out line S, when the interrogation information (in this case a l) and the memory contents in this case a l coincide with each other.

To interrogate whether a 0 is stored or not in cell R 0V is applied to interrogation line D 4V to interro gation line D 4V to word line W, and 0V to word line W However, since the cell R, has been storing a l, the p-channel transistor will be ON, whereas the nchannel transistor OFF. Accordingly, a current flows from interrogation line D, to load resistor L via thepchannel transistor thereby producing an output on the read out line 8,. In this manner, an output appears on the read out line S, when the interrogation information (in this case a O) and the memory content (in this case a l) of cell R, do not coincide with each other. While cell R, is storing a 0, in order to interrogate whether it stores or not a l, a voltage of 4V is ap lied to interrogation line D,, 0V to interrogation line -5V to word line W, and 0V to word line W,. In this case, since a current flows from interrogation line D, through load resistor L connected to read out line S via the nchannel transistor, an output will appear on the read out line 8,. In this manner, an output appears on read out line 8, when the interrogation information and the memory content do not coincide with each other.

While cell R, is storing a 0, when an interrogation is made whether the cell R, is storing ON not a 0 by applying OV to interrogation line D,, 4V to interrogation line D 5V to word line W, and 0V to word line W,, no output will appear on read out line S, because the p-channel transistor is OFF.

As above described, with this arrangement it is possible to check the coincidence and non-coincidence between the memory content of the cell and the interrogation information by the presence or absence of the output on the associative read out line. Furthermore, it can be clearly noted from the above description that the detection can be made concurrently for different word lines.

Subsequent to the associative read out operation described above, an ordinary read out operation is performed. In this case, load resistors (not shown) are connected also to the interrogation lines (digit lines D, and D connected to the first electrodes of the p-channel transistors, a voltage of 4V, for example, is applied to these interrogation lines and 5V is applied to a word line, for example W,, associated with the word to be read out. As above described, since cell R, has been storing a l and since the p-channel transistor is ON, the line D, will assume 0V by being grounded through the load resistor L. Then, when the memory content of cell R is read out by applying -5V to word line W alone, interrogation line D, will assume 4V since the pchannel transistor of cell R is in its OFF state. Thus, it is possible to read out a 1 when interrogation line 2, assumes 0V, whereas a 0 when the interrogation line D, assumes 4y. Similarly, with regard to cell R interrogation line D, will assume 0V whereas with regard to cell R interrogation line D, will assume 4V.

Where a plurality of words are simultaneously selected (when 5V is applied to both word line s W, and W similar output will appear on digit line D, where all bits (in this case R, and R corresponding to respective words are storing l or 0. Cells R and R, and digit lines D and D have the same relationship. Where the bits corresponding to respective words are storing a l and 0 both digit lines D, and D, will assume 0V. This is extremely advantageous in the associative read out. Of course, it is possible to simultaneously read out two bits for each word.

Although in the construction shown in FIG. 1, MOS transistors Ra and Rb are mounted space apart on the same insulator 3 it is possible to form a p-channel MIOS transistor Rb directly on an n-conductivity type semiconductor substrate and to form an n-channel MIOS transistor Ra in a P-type semiconductor region which is formed in the n-conductivity type semiconductor substrate by any well known technique, as shown in FIG. 5. In this modification, 5V is applied to the P- type region, whereas 0V is applied to the N-type region so as to provide an insulation utilizing the p-n junction.

While in the construction shown in FIG. 1, one bit memory cell is constituted by a p-channel MIOS transistor and an n-channel MIOS transistor it should be understood that the one bit memory cell can also be formed by two MIOS transistors of the same conductivity type. More particularly, in the MIOS type transistor, it is possible to form two types of MIOS transistors having gate threshold voltage hysteresis characteristics which rotate in the opposite direction as shown by the arrows in FIG. 6 by controlling the construction of the gate insulations. In this figure, curve IJ represents the hysteresis characteristic of an injection type MIOS transistor, whereas curve ID that of an ion drift type MIOS transistor. In the case of a p-channel, MIOS transistor of the I] type, where the gate voltage V is varied in an order OV(+30V)OV, the threshold voltage Vth will vary along a curve k-l-m-g, whereby the threshold voltage shifts from 10V to 2V. On the other hand, when the gate voltage is varied in an order 0 V(-30- V)0V, the threshold voltage will vary along a curve g-h-i-k thereby shifting the threshold voltage from 2V to lOV.

In the p-channel MIOS transistor of the ID type, when the gate voltage is varied in an order V(+30- V)-0V, the threshold voltage will vary along a curve g-m'-l-k thereby shifting the threshold voltage from 2V to lOV. On the other hand, when the gate voltage is varied in an order 0V(-30V)()V, the threshold voltage will vary along a curve k-i'-k'-g thereby shifting the threshold voltage from V to 2V. As shown in FIG. 6, these two hysteresis characteristics rotate in the opposite directions and vary substantially symmetrically with respect to the ordinate which repre-= sents the value of the threshold voltage Vth. In the case of a p-channel MIOS transistor, impression of a gate voltage of 3OV causes the II type transistor to be ON condition and the ID type transistor to be OFF condition for a gate voltage of SV between the threshold voltage of 2V and the threshold voltage of IOV, whereas impression of a gate voltage of +30V the IJ type transistor to be OFF state and the ID type transistor to be ON state for the same gate voltage of SV.

In the case of an n-channel MOS transistor, when a gate voltage of 3OV is applied, the I] type transistor will be in OFF state whereas the ID type transistor in ON state. On the other hand, application of a gate voltage of +30V causes the I] type transistor to be ON condition whereas the ID type transistor to be OFF condition. Accordingly, where a p-channel MIOS transistor of the II type and a p-channel MOS transistor of the ID type are combined to constitute a one bit memory cell it is also possible to form an associative memory by making the ON state of the IJ type transistor and the OFF state of the ID type transistor to correspond to a binary l and by making the OFF state of the I] type transistor and the ON state of the ID type transistor to a binary 0.

FIG. 7 shows a connection diagram of an associative memory comprising 16 memory cells each including a p-channel MIOS transistor (II) of the II type and a pchannel MIOS transistor (ID) of the ID type. The gate electrodes 9 of transistors of memory cells P through P are commonly connected to a word line W the gate electrodes of transistors of cells P through P to a word line W the gate electrodes of transistors of cells P through P to a word line W and the gate electrodes of transistors of cells P through P to a word line W The first electrodes 4 of ID type transistors of cells P P P and P are connected to a digit line D while the first electrode s 4 of I] type transistors of the same cells to a digit line D The first electrodes of ID type transistors of cells P P P and P are connected to a digit line D;, while the first electrodes 4 of IJ type transistors of the same cells to a digit line D The first electrodes of ID type transistors of cells P P ,P, and P are connected to a digit line D, and the first electrodes of II type transistors of the same cells to a digit line D The first electrodes of ID type transistors of cells P P P and P are connected to a digit line D, whereas the first electrode of II type transistors of the same cells to a digit line D.,. The second electrodes 5 of the MIOS transistors associated with respective word lines W through W are connected to associated read out lines 8, through 8,, respectively.

To write informations in this memory device, +30V, for example, is impressed upon respective word lines W, through W to write 0 in all cells constituted by cells P, through P thereby turning OFF the transistors of the II type and ON the transistors of the ID type. Then,

to write a l in a cell P a 0 in a cell P a l in a cell P and a 0 in a cell P a potential of 3OV is impressed upon the digit linesD D D and D and the word line W Then, although the II type transistors and the ID type transistors of cells P and P are maintained in their OFF state and ON state respectively, in other words, the memory content 0 is preserved, the IJ type transistors of cells P and P are inverted to ON state whereas those of the ID type of the same cells are inverted to OFF state thereby changing to a I state. In this manner l-O-l-O are written in. In this embodiment, it can be clearly noted that informations are written in various cells corresponding to word lines W W and W according to the same order. In other words, a voltage of +30V is impressed upon all word lines W through W FIG. 8 shows a connection diagram for interrogating informations l-O-0-l for the informations 1-0-1-0 written in the memory shown in FIG. 7. More particularly, word lines W through W are connected in common, whereas all read out lines 8, through 5., are grounded respectively through load resistors L. Generally, to interrogate a O 2 1 negative potential is impressed upon D lines whereas D lines are grounded. To interrogate a l, D lines are grounded whereas D lines are impressed with a negative potential. Accordingly, -5V is impressed upon the word lines, 4V upon digit lines D 52, E3 and D4 and lines 51, D2, D3 and 5 4. Since informations l have been stored in cells P P P and P respectively, the ID type transistor (shown on the left hand side) are in their OFF state whereas the transistors of the IJ type (shown on the right hand side) are in their ON state for the gate voltages of 5V. Same conditions hold in other cells P P P and P Since cells P P P and P have been stored 0 respectively, the transistors of the ID type are in ON state whereas the transistors of the II type are in OFF state for the gate voltage of 5V. Same conditions also hold in cells P P P and P respectively.

Assuming now tl 1 at4V and 0 are impressed only on digit lines D, and D respectively, since the interrogation information I and the memory content 1 coincide with each other, no output appears on the read out line S Similarly, when 0 and 4V are applied only upon digit lines D and D respectively, there is no output on the read out line. when 0 V and 4V are impressed upon digit lines D and D respectively, since the inter rogation information 0 and the memory content 1 do not coincide with each other, an output appears on the read out line S In the same manner, an output appears on a read out conductor when 4V and 0V are impressed upon digit lines D, and D respectively. When voltages shown in FIG. 8 are impressed upon respective digit lines, since the interrogation informations l-O-O-l and the memory contents l-O-l-O do not coincide with each other, an output appears on the read out line S With the modification shown in FIG. 8, it is possible to detect the coincidence or non-coincidence of the interrogation informations and the memory contents by comprising them, just in the same manner as in the embodiment shown in FIG. 4.

FIG. 9 shows a connection diagram of a modified associative memory in which words W alone are read out simultaneously from the memory contents stored in the memory shown in FIG. 7. In this case, read out lines 8,, S S and 8,; are all grounded and respective digit lines are connected to a common source of -4V, respectively, through load resistors L.

Again, it is possible to determine the contents of the cells P through P by detecting the voltages appearing on respective digit lines in the same manner as has been described in connection with FIG. 4.

As above described, the invention provides an im-.

proved associative memory characterized in that each one bit associative memory cell is constituted by two MIOS transistors, that the memory is non-destructive, that a single line can be used as a digit line as well as an interrogation line, and that simultaneous read out of a plurality of words does not destroy the memory contents of other words.

It is to be understood that the voltages applied for performing writings, associative read outs and ordinary read outs, and the number of one bit memory cells are not limited to the particular values illustrated in the embodiments.

What we claim is:

1. An associative memory comprising one bit memory cell including:

a pair of MIOS transistors each having a source electrode, a drain electrode, and a gate electrode including double insulating layers and presenting threshold voltage-hysteresis characteristics;

a word line connected commonly to the gate electrodes of said pair of MIOS transistors;

a pair of digit lines connected respectively to the drain electrodes of said pair of MIOS transistors;

a read-out line connected commonly to the source electrodes of said pair of MIOS transisotrs;

writing means for writing a binary signal, including means for selectively impressing voltages of different levels respectively on said pair of digit lines according to the content of said binary signal and for selectively impressing at the same time on said word line a negative or positive polarity voltage of a level high enough to vary the hysteresis characteristics of said pair of MIOS transistors also according to the content of said binary signal; and

interrogation means for detecting an output from said read-out line, including means for selectively impressing voltages of different levels respectively on said pair of digit lines according to the content of a binary signal to be interrogated and for impressing at the same time on said word line a voltage of intermediate value between the threshold voltages presented by said hysteresis characteristics varied by said writing means.

2. An associative memory as claimed in claim 1 wherein said pair of MIOS transistors comprise a combination of a p-channel MIOS transistor and an nchannel MIOS transistor and wherein said p-channel and n-channel MIOS transistors present the threshold voltage hysteresis characteristics in the same rotating direction for the voltage impressed on said word line when said writing means is conducted.

3. An associative memory as claimed in claim 1 wherein one of said pair of MIOS transistors is of the injection type whereas the other is of ion drift type having the same conductivity type as that of said injection type.

4. An associative memory as claimed in claim 1 wherein each transistor constituting said pair of MIOS transistors has a gate of metal nitride oxide structure.

5. An associative memory according to claim 1 wherein a plurality of said one bit memory cells are arranged in a matrix, the gate electrodes of said transistors constituting said one bit memory cells in the same row are connected to a common word line, the source electrodes of said transistors are commonly connected to an output line of said associative read-out means, the drain electrodes of one of said two MIOS transistors constituting one bit cells in the same column are commonly connected to one digit line, and the drain electrodes of the other MIOS transistor are commonly connected to another digit line. 

1. An associative memory comprising one bit memory cell including: a pair of MIOS transistors each having a source electrode, a drain electrode, and a gate electrode including double insulating layers and presenting threshold voltage-hysteresis characteristics; a word line connected commonly to the gate electrodes of said pair of MIOS transistors; a pair of digit lines connected respectively to the drain electrodes of said pair of MIOS transistors; a read-out line connected commonly to the source electrodes of said pair of MIOS transisotrs; writing means for writing a binary signal, including means for selectively impressing voltages of different levels respectively on said pair of digit lines according to the content of said binary signal and for selectively impressing at the same time on said word line a negative or positive polarity voltage of a level high enough to vary the hysteresis characteristics of said pair of MIOS transistors also according to the content of said binary signal; and interrogation means for detecting an output from said read-out line, including means for selectively impressing voltages of different levels respectively on said pair of digit lines according to the content of a binary signal to be interrogated and for impressing at the same time on said word line a voltage of intermediate value between the threshold voltages presented by said hysteresis characteristics varied by said writing means.
 2. An associative memory as claimed in claim 1 wherein said pair of MIOS transistors comprise a combination of a p-channel MIOS transistor and an n-channel MIOS transistor and wherein said p-channel and n-channel MIOS transistors present the threshold voltage hysteresis characteristics in the same rotating direction for the voltage impressed on said word line when said writing means is conducted.
 3. An associative memory as claimed in claim 1 wherein one of said pair of MIOS transistors is of the injection type whereas the other is of ion drift type having the same conductivity type as that of said injection type.
 4. An associative memory as claimed in claim 1 wherein each transistor constituting said pair of MIOS transistors has a gate of metal nitride oxide structure.
 5. An associative memory according to claim 1 wherein a plurality of said one bit memory cells are arranged in a matrix, the gate electrodes of said transistors constituting said one bit memory cells in the same row are connected to a common word line, the source electrodes of said transistors are commonly connected to an output line of said associative read-out means, the drain electrodes of one of said two MIOS transistors constituting one bit cells in the same column are commonly connected to one digit line, and the drain electrodes of the other MIOS traNsistor are commonly connected to another digit line. 